The NZ Transport Agency did not adequately assess other options before deciding to push ahead with a flyover, a board of inquiry has heard.

The assessment of other traffic solutions at the roundabout, particularly the addition of an extra lane, were not considered robustly enough, and if they had been a different outcome may have been reached, traffic and highway engineer David Young said when he took the stand today.

He appeared before the four person board deciding whether to grant resource consent for NZTA's proposed Basin $90 million flyover 20 metres north of the Basin Reserve.

Mr Young is appearing on behalf of opposition groups Save the Basin and the Mt Victoria Residents' Association.

He told the board the "tick the box" assessment system used by NZTA to rule out options did not adequately assess cost-benefits of the flyover compared to other options.

In particular, the "do minimum" should not have been taken as the status quo, but rather option to add a third lane around the cricket ground, the option known as the Basin Reserve Roundabout Enhancement Option (Brreo).

"That should have been treated as the do minimum, what we now call Brreo. That would have given some quite different answers."

Brreo would widen the roundabout to a minimum of three lanes and is a more refined version of architect Richard Reid's Option RR proposal, which Wellington City Council spent $40,000 investigating last year. He claims it is a superior congestion solution.

Lawyer Philip Milne, representing the Architecture Centre and Newtown Residents Association, also questioned Mr Young about whether more consideration should have been given to tunnel options, such as the cut and cover solution Option X advocated by the Architecture Centre.

Option X involved a cut-and-cover tunnel from Sussex St to Taranaki St, and pedestrian bridges at the Sussex/Buckle St intersection.

NZTA has put the potential cost of the alternative at up to $255m.

Mr Young said he understood Option X was ruled out because it was too expensive, but a robust assessment of the cost benefit had not been carried out and the negative impacts of a bridge were not properly accounted for.

"I would have thought that would have been dealt with a little bit more robustly than 'no we can't afford it'," he said.

However, he did not think that the Memorial Park Underpass currently being constructed would have made a difference to the outcome of a robust assessment.

Mr Milne has questioned whether Option X was adequately re-assessed in light of that project being announced.